Near-field scanning optical microscope having a sub-wavelength aperture array for enhanced light transmission

ABSTRACT

A metallic film has apertures located therein in an array arranged in a pattern so that when light is incident on the apertures, surface plasmons on the metallic film are perturbed resulting in an enhanced transmission of the light emitted from individual apertures in the array. The aperture array is used: to filter light of predetermined wavelength traversing the apertures, to collect light over a distance after traversing the apertures, to improve operation of near-field scanning optical microscopes, and to enhance light transmission through masks useable in photolithography.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.08/979,432, filed Nov. 26, 1997, now U.S. Pat. No. 5,973,316 issued Oct.26, 1999, and claims the benefit of U.S. Provisional Patent ApplicationSer. No. 60/051,904, filed Jul. 8, 1997.

FIELD OF THE INVENTION

The present invention relates to optical transmission throughsub-wavelength aperture arrays and particularly relates to improvedtransmission of light through such apertures.

BACKGROUND OF THE INVENTION

There has been great interest to control and use light, or photons, inthe same way that electrons are put to use in solids to make all typesof electronic devices and provide communication between distant points.The development of fiber optics and semiconductor lasers haverevolutionized the telecommunications industry. Optics is at the heartof the fabrication of integrated circuits, data storage, compact discs,and so forth. However, while the use of light has already demonstratedmany advantages, there are considerable challenges ahead to fully makeuse of its potential. For example, it is desirable to engineer materialsso that the propagation of light occurs only in certain directions forcertain frequencies (i.e. photonic band gap materials). Efforts in theseareas have led to translucent dielectric materials, known as photonicband gap materials or structures, which are opaque at certainfrequencies. Light modes cannot propagate through the materials if theirfrequencies are within those defined by the band gap. The limitations ofsuch materials is that they will permit light transmission at nearbyfrequencies. That is, these materials only block a narrow range offrequencies within a broad spectrum. Optical detectors are normallysensitive to a broad spectrum of light so that light of slightlydifferent frequencies from that which is blocked might get through tothe detectors and be detected. Therefore it would be much more useful tohave a material or a device that operates exactly in a reverse manner,that is, it selectively transmits light only in a narrow range offrequencies within a broad spectrum.

The present invention has application in many areas, including thosewhere the divergence of a light beam is a problem, or where increasedlight transmission through an aperture array is desirable, or inphotolithography, or in optical filtering applications.

Opto-electronics, for example, is concerned with optical inter-chipcommunication in order to increase computation speed. The chips areusually all located on the same board and efforts are made to integrateoptical lasers emitting from one chip to detectors located on otherchips. One of the difficulties encountered is to make sure the emittedlight does not diverge but rather remains collimated over sufficientlylong distances so that the light reaches the desired detector or so thatthe light can be input into a fiberoptic bundle where each fiber isconnected to a preselected chip or destination. As the structures andbeam sizes approach that of the wavelength of light, the divergence andtransmission of the light become even greater problems.

Also, conventional microscopes, and generally optical imaging andstorage devices which operate in the far field, cannot resolve featuressubstantially smaller than about one-half the wavelength of the lightused. In order to overcome this resolution problem, near-field scanningoptical microscopes (NSOM) were developed where an aperture much smallerthan the wavelength of the probing light is placed near the specimen andscanned over its surface. A fraction of the light passing through thespecimen is then collected through the aperture and relayed to aphotodetector. Alternatively, light passes through the aperture, throughthe specimen and is then captured by a photodetector. The image of thespecimen is then reconstructed by combining the signal at thephotodetector with the microscope position over the specimen. However,the problem with an aperture smaller than the wavelength of light isthat its transmissivity decreases rapidly and is proportional to theradius of the aperture divided by the wavelength to the power 4, i.e.(d/λ)⁴. As a result, much effort has gone into designing betterapertures, such as fiber tips. However the transmission efficiency ofthese apertures is still orders of magnitude less than the optimalefficiency.

In another case, the resolution of photolithography, which is central tothe chip manufacturing industry, is also limited in resolution toapproximately one-half of the wavelength of the incident light.Techniques such as near-field scanning microscopy can be used to createsmaller patterns in the photoresist, however such techniques aregenerally extremely slow since the photoresist patterns must be writtenon every chip. Unlike the case for traditional photolithography, thepatterns cannot be projected through a mask, the standard industrialtechnique. In addition, as discussed above, the light transmissionefficiency through smaller-than-wavelength apertures, such as taperedoptical fibers, is very small. This slows the process even more becausea minimum amount of light must impinge upon the photoresist in order tochange its characteristics.

In another application, filters made from wire-mesh or metallic gridshave been used extensively for filtering light in the far IR (infrared)spectrum (e.g. 10˜800 micrometer wavelengths). These filters comprisethin metallic wires (much thinner than the wavelengths to betransmitted) deposited on an optically clear support. The filters arecharacterized by a transmission spectrum having a peak at approximately1.2 times the periodicity of the mesh. The peak is very broad, typicallygreater than half the periodicity of the mesh. Mesh filters have beenextensively studied and their properties have been explained by analogywith transmission line circuits. These filters would be much more usefulif their transmission spectra could be narrowed to make them moreselective.

The main object of the present invention is to overcome the problems andlimitations described above by transmitting light very efficientlythrough an array of apertures, where each aperture is much smaller thanone-half of the wavelength of the light and by allowing lighttransmission only at certain frequencies of light which can becontrolled by the structure and arrangement of the aperture array.

SUMMARY OF THE INVENTION

The present invention provides an array of subwavelength apertures in ametallic film or thin metallic plate, each aperture having a diameter dand inter-aperture spacing P. The light transmission properties of suchan apparatus are strongly dependent upon the wavelength of the light.Enhanced transmission occurs for light wavelengths in relation to thespacing P. The enhanced transmission is greater than the unittransmission of a single aperture and is believed to be due to theactive participation of the metal film in which the aperture array isformed.

If the apertures are viewed, as has previously been the case, as merelygeometrical openings, then the transmission efficiency (defined as thetransmitted intensity divided by the intensity of the light directlyimpinging onto the aperture) would be less than one percent. However, inthe present invention, as will be described below, the transmissionefficiency is greatly increased and the resulting apparatus may beconsidered as a near-field probe, with the added capability of tailoringthe transmission properties of the light at a desired wavelength byadjusting the spacing or periodicity P.

The practical effect of this result is the application of the inventionin fields such as wavelength selective optical filtering (particularlyin the visible and near-infrared), light beam collecting, near-fieldscanning optical microscopy and photolithography.

A principal object of the present invention is therefore, the provisionof a novel apparatus including an aperture array for enhancing thetransmission of light therethrough.

Another object of the invention is the provision of a metal film havingan array of apertures for enhanced transmission of light therethrough.

Further and still other objects of the present invention will becomemore clearly apparent when the following description is read inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view of an aperture array in a thin filmmaterial; and

FIG. 2 is a graphical representation of the zero-order transmissionspectrum for a rectangular array of apertures in a metal film;

FIG. 3 is a graphical representation of transmission intensity as afunction of wavelength, normalized to the period P, for different metalsand aperture array geometries;

FIG. 4 is a graphical representation of the angular dependence of theincident light on the transmission of light through an aperture array;

FIG. 5 is schematic representation of an optical filter;

FIG. 6 shows an aperture array used in a light collector;

FIG. 7A is a representation of a near-field optical scanning microscopetip relative to a specimen and light source;

FIG. 7B is an expanded view of the frontal surface of the tip in FIG.7A;

FIG. 7C is a representation of a near-field optical scanning microscopetip relative to a specimen and photodetector;

FIG. 8A shows an aperture array on a metal film used forphotolithography;

FIG. 8B shows a substrate with lines written thereon;

FIG. 9 is a graphical representation of the transmission efficiency as afunction of wavelength for a thin film material having parallel slitstherein;

FIG. 10A is a plan view of rectangular aperture array without the outeraperture;

FIG. 10B is a graphical representation of the transmission intensity asa function of wavelength for the aperture array in FIG. 10A;

FIG. 11A is a plan view of an alternative aperture array pattern; and

FIG. 11B is a graphical representation of the transmission intensity asa function of wavelength for the aperture array in FIG. 11A.

DETAILED DESCRIPTION

Referring now to the figures and to FIG. 1 in particular, there is shown(not to scale) a thin metal plate or thin metal film 10 containing arectangular array of cylindrical apertures 12. The metal may be anymetal and is preferably Al, Ag, Au or Cr. The diameter of an aperture isd and the periodicity or spacing between apertures is P. The thicknessof the metal film or metal plate 10 is preferably in the range ofapproximately 0.05 to 10 times the aperture diameter. The intensity ofthe incident light is I_(INCIDENT) and the intensity of the light aftertraveling through the apertures is I_(OUTPUT). In FIG. 1 an unsupportedthin metal plate is shown, however, a thin metal film deposited on asubstrate, such as a glass or quartz, is also contemplated by thepresent invention. While the apertures are shown as round, they may haveother shapes, e.g. oval or rectangular. While the array is shown as arectangular array, other aperture array configurations are alsopossible, such as triangular, without deviating from the teachings ofthe invention.

The aperture arrays exhibit distinct zero-order transmission spectrawith well-defined peaks. The maxima occurs at wavelengths approximately10 times the diameter, d, of the individual aperture 12. Thetransmissivity is much greater than that expected from conventionaltheory. Our experiments indicate that the unusual optical properties areprobably due to the coupling of the incident light with the surfaceplasmons of the periodic rectangular aperture array in the metal. Thecoupling becomes extremely strong at wavelengths of incident lightlarger than the period P. From the peak positions of the transmissionspectra as a function of incident angle, dispersion curves are obtainedwhich reflect the structure of a surface plasmon dispersion. It isbelieved that the array of apertures sufficiently perturb the propertiesof the metal to result in a well defined surface plasmon energy bandstructure with gaps. It is this surface plasmon energy band that webelieve is responsible for the enhanced transmissitivy through theindividual apertures. It is possible that other phenomena, such asdiffraction or interference due to array geometry, also contribute tothe enhanced transmission.

In a modification of the structure shown in FIG. 1, two-dimensionalarrays of cylindrical apertures in metallic films were prepared andanalyzed. For example, a silver film of thickness t=0.2 μm was depositedby evaporation on a fused quartz or glass substrate. The apertures werefabricated through the film by sputtering using a Micrionfocused-ion-beam (FIB) System 9500 (50 keV Ga ions, 5 nm nominal spotdiameter). The individual hole diameter, d, was varied between 150 nmand 1 μm and the spacing between holes in the rectangular array, P, wasbetween 0.6 and 1.8 μm. The zero-order transmission spectra wererecorded with a Cary 2 UV-NIR spectrophotometer with an incoherent lightsource. Additional studies of transmission, diffraction and reflectionproperties where performed on an optical bench using coherent lightsources.

FIG. 2 shows a typical zero-order transmission spectrum for arectangular array of 150 nm apertures with a periodicity of 0.9 μm in a200 nm thick Ag film. The spectrum shows a number of distinct features.At 326 nm, the narrow bulk silver plasmon peak is observed whichdisappears as the film become thicker. This is an unexpected result andis believed to be the result of resonant excitation of a local plasmonmode. The peaks become gradually stronger at longer wavelengths,increasingly so even beyond the spacing, P.

At long wavelengths there is no diffraction from the aperture array orfrom individual apertures. The first-order diffraction spots can be seento be grazing the surface as the wavelength increases towards thespacing P. The maximum transmitted intensity occurs at 1370 nm, nearlyten times the diameter of an individual hole in the array. The absolutetransmission efficiency, calculated by dividing the fraction of lighttransmitted by the fraction of surface area occupied by the holes, isgreater than 2 at the maximum. That is, more than twice as much light istransmitted from the aperture array than impinges directly on theapertures. Moreover, the transmissivity of the aperture array increaseslinearly with the surface area of the holes.

This result is contrary to the expected result that transmission of asingle subwavelength aperture is predicted to scale as (d/λ)⁴, so thatfor a 150 nm diameter hole, the expected transmission efficiency is onthe order of 10⁻². See, H. A. Bethe, Theory of Diffraction by SmallHoles, Physical Review 66, 163-82, 1944. In addition, the intensity ofthe zero-order transmission from a grating is expected to decreasemonotonically at longer wavelengths (I˜1/λ). See, M. Born & E. Wolf,Principles of Optics, Pergamon Press, Oxford 1980. Therefore, thepresent invention yields results indicative of the aperture array itselfbeing an active element and not merely a passive geometrical objectlocated in the path of an incident beam of light.

FIG. 3 shows curves of transmission intensity as a function ofwavelength/period for different metals. The solid line is 200 nm Agfilm, aperture size of 150 nm and spacing between apertures of 0.6 μm,the dashed line is 300 nm Au film, aperture size of 350 nm and spacingbetween apertures of 1.0 μm and the dashed-dotted line is 100 nm Crfilm, aperture size of 500 nm and spacing of 1.0 μm. The peaks occur inrelation to the spacing between apertures, independent of the metal(e.g. Al, Ag, Cr, Au), aperture diameter and film thickness. The widthof the peak is strongly dependent on the aspect ratio (t/d or metal filmthickness divided by aperture diameter) of the cylindrical holes. Fort/d=0.2, the peaks are very broad and when the ratio approaches unity,the maximum sharpness is obtained. The spectra change significantly withthe geometric configuration of the apertures, e.g. when the array issquare or triangular. The spectra are identical whether the illuminationis from the metal side or the substrate side of the array.

The enhanced transmission spectra is dependent upon the angle of thelight incident upon the aperture array. FIG. 4 shows the spectrameasured for 2 degree changes of incident angles for angles between 0and 20 degrees. The peaks change in intensity and split into new peakswhich move in opposite directions.

The angular dependence effect results in a novel wavelength selectivefilter as shown in FIG. 5. By adjusting the angle θ of support 50, andhence the angle of aperture array 52, the wavelength at which the lightpeaks as a function of the angle is as shown in FIG. 4. By using thisproperty, a filter comprising an aperture array in a metal film,adjusted for a predetermined incidence angle can be formed. The filtercan be used for ultraviolet, visible and longer wavelengths. Theadvantage of the arrangement is that only zero-order light istransmitted and only light at a wavelength corresponding to the incidentangle as measured by angle θ.

The aperture arrays are more wavelength selective as filters thanconventional mesh array. Moreover, unlike photonic band gap arrays wherethe material is passive and translucent at all wavelengths except at theenergies within the gap, the present invention provides a material thatis opaque at all wavelengths except those for which coupling occurs.

Another application of the invention is as a light beam collector asshown in FIG. 6. Light 60 is incident on cylindrical apertures 61 inmetal film 62 deposited on substrate 63. After traveling through thearray of apertures 61, the collected light travels to optic fibers in abundle or array of fibers 65 in juxtaposition to the apertures. Theintensity of the light passing through the aperture array and enteringinto the fibers is enhanced by the present invention. The aspect ratioof the apertures is not critical but the spacing P between apertures isimportant for determining the wavelength of the enhanced lightcollection as shown in FIG. 6. Previously, it was difficult to directlight into subwavelength fibers. Complex lenses and alignment deviceswere used to direct the light. Since in the present invention theapertures transmit more light than the aperture surface area, the metalaperture array acts like a light collector and hence, coupling lightinto subwavelength fibers is efficient.

Another application for the aperture array is in near-field scanningoptical microscopy. FIG. 7A shows the general arrangement used innear-field scanning optical microscopy where a light source 70 transmitslight through a specimen 71 supported by a support into a scanning tip72 which acts as a light collector. In accordance with the teachings ofthe present invention, the frontal surface 73 of the tip 72 includes twoor more subwavelength apertures 74 (FIG. 7B) in a metallic film coatingon the surface 73 for increasing the intensity of the light received bythe probe for subsequent conventional NSOM signal processing. Thisconfiguration is most effective when the coating is Ag and a He--Cdlaser is the light source. In FIG. 7C the probe acts as the light sourceand transmits light from the probe tip 75 via apertures 76 in thefrontal surface 77, through the specimen 71, to a photodetector 78 forconventional signal processing. The inclusion of at least two apertures76 in the frontal surface 77 of the probe tip 75 in FIG. 7C increasesthe light transmitted from the probe tip 75 into the specimen forsubsequent collection at photodetector 78.

In another application of the invention shown in FIG. 8A, the hightransmission efficiency of the aperture array can be used to make novelmasks for sub-wavelength photolithography by virtue of the arrangementof very small apertures in a metallic mask. Light 80 having a wavelengthλ, much greater than the diameter of the apertures, can be projectedthrough the apertures 81 in a metal plate or metal film deposited on asubstrate 82 to yield lithographic structures having features muchsmaller than λ/2.

In order to image a two-directional subwavelength line width, which forillustrative purposes is shown in the form of the letter H, ontosubstrate 83 coated with photosensitive material as shown in FIG. 8A,the apertures 81 are separated a distance dependent on the wavelength ofthe light to "image" an x-direction and a y-direction line in thephotosensitive material coated on substrate 83. In order to write a linein the x-direction light polarized in the x-direction is transmittedthrough the aperture array. Light polarized in the x-direction will notpass through lines in the y-direction. The mask and substrate are madeto undergo relative translatory motion of one-half the period or spacebetween apertures in the x-direction by use of actuator 84. Since thefilm only has apertures, lines can be written only if the apertures aremoved along a linear path relative to the substrate 83. The procedure isthen repeated for lines in the y-direction using y-direction polarizedlight and actuator 85. FIG. 8B shows the substrate 83 with the letter Hwritten into the photoresist coating.

In an alternative photolithography arrangement, the apertures 81 in thethin film metallic mask are replaced by parallel slits. FIG. 9 is agraph of the transmission efficiency (I_(OUTPUT) /I_(INCIDENT)) as afunction of wavelength for a thin aluminum film having parallel slots0.15 μm wide by 40 μm long spaced 0.6 μm apart. The incident light waspolarized at 90 degrees in the plane transverse to the longitudinal axisof the slit. The mask containing slits and light are used in the samemanner as the apertured mask. Of course, by using a slit of sufficientlength, it may be possible to avoid the necessity of moving the maskrelative to the substrate.

In a further alternative arrangement, the apertures are located in themetal film in the pattern of Fresnel zone lenses to focus the light atthe focal points of the lenses. The mask containing the apertures willundergo motion as described in conjunction with the aperture mask inFIG. 8A. The aperture pattern in the film resembles a rectangular array,but the apertures are only present at those locations of the rectangulararray corresponding to a Fresnel zone lens pattern.

The use of the aperture array as described for photolithography allowssubwavelength lines to be written without using deep-UV and X-raysources while at the same time permitting the use of conventionalphotoresists.

Variations and modifications of the aperture array are possible withoutmaterially affecting the light transmission through the apertures. Forexample, FIG. 10A shows a square array of 25 apertures 100 in a thinmetallic film 102 in which one aperture, the center aperture, ismissing.

FIG. 10B is a graph of the transmission intensity as a function ofwavelength for an array of 0.5 μm diameter apertures spaced 1 μm apartin 0.1 μm thick chrome film, with one aperture, the center aperture,missing from the array.

In FIG. 11A a repeated pattern of groups 110 of seven apertures 112 inthin metallic film 114 are spaced apart by a distance D. FIG. 11B is agraph of the transmitted intensity as a function of wavelength for anarray of apertures as shown in FIG. 11A where the apertures have adiameter of 0.5 μm, spacing of 1 μm and the groups are separated by adistance D of 5 μm in a chrome film having a thickness of 0.1 μm.

The graphs show that arrays having missing apertures or array ofaperture patterns other than a rectangular array yield similar resultsto square aperture arrays.

While the figures show the light being incident to the metal film sideof a substrate having a metal coating on one side thereof, similarresults are achieved when the light is incident on the substrate side,opposite the metal film.

While there has been described and illustrated an aperture array in ametallic film or thin metallic plate for use in certain applications, itwill be apparent to those skilled in the art that variations andmodifications are possible without deviating from the spirit and broadteachings of the invention which shall be limited solely by the scope ofthe claims appended hereto.

What is claimed is:
 1. A near-field scanning optical microscopecomprising:a light source; a probe having a frontal surface disposedrelative to said light source for receiving light from said lightsource; said frontal surface having a metallic film containing an arrayof apertures therein disposed thereon, said apertures in the array beingspaced apart by a distance P, where P is selected dependent upon thewavelength of light incident on said array, whereby incident light at apredetermined wavelength will perturb the metallic film in a surfaceplasmon energy band for enhancing transmission of light throughindividual apertures in said array; and photodetector means forreceiving light traversing said metallic film.
 2. A near-field scanningoptical microscope comprising:a light source; a probe including afrontal surface having a metallic film containing an array of aperturestherein disposed thereon coupled to said light source for emitting lightfrom said apertures, said apertures in the array being spaced apart by adistance P, where P is selected dependent upon the wavelength of lightincident on said array, whereby incident light at a predeterminedwavelength will perturb the metallic film in a surface plasmon energyband for enhancing transmission of light through individual apertures insaid array; and detector means disposed relative to said frontal surfacefor receiving light emitted from said apertures.
 3. A near-fieldscanning optical microscope as set forth in claim 1, where said metallicfilm is Ag.
 4. A near-field scanning optical microscope as set forth inclaim 1, where said light source is a He--Cd laser.
 5. A near-fieldscanning optical microscope as set forth in claim 1, where said array ofapertures comprises two or more subwavelength apertures.
 6. A near-fieldscanning optical microscope as set forth in claim 2, where said metallicfilm is Ag.
 7. A near-field scanning optical microscope as set forth inclaim 2, where said light source is a He--Cd laser.
 8. A near-fieldscanning optical microscope as set forth in claim 2, where said array ofapertures comprises two or more subwavelength apertures.